1. Field of the Invention
The present invention generally relates to an optical signal processing apparatus, and more particularly, a wavelength conversion device converting a high speed signal light to a given wavelength in a wide wavelength range by using optical four wave mixing.
In an all-optic network or a photonic network where signal lights of various wavelengths are used, a technology of converting one wavelength of a signal light to another wavelength is desired for preventing collision among signal lights of different channels, for achieving reuse of vacant wavelengths, or free connection between sub-networks.
2. Description of the Related Art
As a wavelength conversion technology using an all-optic process employing no photoelectric conversion of optic signals, an optical four wave mixing technology using a nonlinear medium is known.
FIG. 1 shows an example of wavelength conversion by optical four wave mixing using a traveling wave type semiconductor optical amplifier having a multiple quantum well (MQW) structure as a nonlinear medium.
With reference to FIG. 1, the semiconductor optical amplifier is formed on an n+ type InP substrate 11, and includes an MQW active layer 12 having an InGaAs/InGaAsP structure being formed on the substrate 11, and a p+ type InP clad layer 13 being formed on the active layer 12. The n+ type substrate 11 is grounded, and a positive voltage is applied to the InP clad layer 13. As a result, the active layer 12 is in a gain state.
In the example of FIG. 1, a signal light 14 having an angular frequency ωs and an excitation light 15 having an angular frequency ωp are incident to one end of the semiconductor optical amplifier. When the signal light 14 and the excitation light 15 enters the biased optical amplifier in a gain state, other than the signal light 14 having an angular frequency ωs and the excitation light 15 having an angular frequency ωp, a phase conjugate wave 16 having an angular frequency of 2ωp−ωs is also obtained by optical four wave mixing, as shown in FIG. 2.
This is a phenomenon created in a case where the signal light 14 having an angular frequency of ωs (≠ωp) is incident to the MQW active layer 12 of the semiconductor optical amplifier excited by the excitation light 15 having an angular frequency ωp, in which two photons in the excitation light 15 and one photon in the signal light 14 interact to generate the phase conjugate wave 16 having a frequency of 2ωp−ωs.
Thus, the phase conjugate wave 16 having a frequency of 2ωp−ωs alone can be extracted by disposing a filter that removes the signal light 14 having an angular frequency ωs and the excitation light 15 having an angular frequency ωp at an outgoing end of the semiconductor optical amplifier of FIG. 1.
In the optical four wave mixing phase conjugate wave generation apparatus using such semiconductor optical amplifier, since a semiconductor active layer being an optic gain medium, that is, the MQW active layer 12 is used as a nonlinear type medium, a large nonlinear type effect can be attained, and a converted light in an active layer with a length of several 100 μm can be obtained.
However, with the conventional wavelength conversion device using optical four wave mixing, a conflict between the effect of CDP (Carrier Density Pulsation) and the effect of SHB (Spectral Hole Burning) occurs in a case where a semiconductor gain medium injected with a carrier, that is, a bulk semiconductor or a quantum well semiconductor is employed as a nonlinear type medium that causes optical four wave mixing. Depending on whether the difference in angular frequency between the excitation light 15 and the signal light 14 (ω=ωp−ωs) is positive or negative, that is, whether the angular frequency ωs of the signal light 14 is more toward short wavelength or long wavelength with respect to the angular frequency ωp, the conflict causes a problem where conversion efficiency may become asymmetric, particularly, conversion efficiency from short wavelength to long wavelength may be deteriorated.
It is to be noted that “carrier density pulsation effect” is a phenomenon where wavelength difference, that is, angular frequency difference of incident laser light generates a beat in an electric field, and results to generation of a beat in the carrier density, thereby altering the carrier density. Meanwhile, “spectral hole burning effect” is an effect where a third order nonlinear polarization is created by virtual absorbing of light, in which a dip is created at a position of an oscillation wavelength in a gain spectrum of a semiconductor laser to reduce the gain.
That is, the conversion efficiency x of the optical four wave mixing phase conjugate wave generation apparatus is expressed as:χ∝|ξ(3)(λs)|2·G(λs)·P2pumpin a case of employing a third order nonlinear susceptibility ξ(3) of the third order nonlinear effect, and a susceptive linear gain G of the signal light 14, wherein λs is the wavelength of the incident signal light 14, and wherein Ppump is the power of excitation light 15. As a result of superposing the term corresponding to the carrier density pulsation effect and the term corresponding to the spectral hole burning effect, in a nonlinear medium having a typical quantum well structure, the third order nonlinear susceptibility has a property of being large when the wavelength of the signal light 14 is more toward a long wavelength (ω>0), and being small when the wavelength of the signal light is more toward a short wavelength (ω<0) with respect to the oscillation wavelength. The conversion efficiency x is affected by this property.It is to be noted that |ξ(3)(λs)|2 expresses a second power of the absolute value of ξ(3) (λs).
In Japanese Laid-Open Patent Application No. 11-326964, using a zero order carrier confining quantum structure, for example, a quantum dot, as a semiconductor gain medium is proposed for solving the problem of asymmetry of the conversion efficiency x. The zero order carrier confining quantum structure, such as quantum dot, forms a pseudo two level system, in which change of refractive index becomes zero in a resonance frequency in a same manner as a two level system, and hardly no modulation of refractive index due to the carrier density pulsation effect occurs.
Today there are various known techniques for forming a quantum dot.
For example, Japanese Laid-Open Patent Application No. 9-064476 or Japanese Laid-Open Patent Application No. 9-326506 shows a technology where hetero-epitaxial structure forming a strain with respect to a substrate is repeatedly stacked one on top of the other with an intermediary layer sandwiched therebetween, and using an island like area formed from self-organization by the so-called S-K mode (Stranski-Krastanow) as the quantum dot. Furthermore, Japanese Laid-Open Patent Application No. 2002-141548 describes a technology of forming a quantum dot on an apex part of an etch pit of a tetrahedron formed in a semiconductor layer.
Meanwhile, in a case of actually applying a wavelength conversion device using a nonlinear medium including quantum dot to a wideband photonic network, an optical four wave mixing wavelength conversion is to be executed with respect to a signal light of various wavelengths. Although an excitation light source is to be provided for each signal light wavelength used in the photonic network, such configuration increases the scale of the device, and raises cost considerably.